ajflate is a specialized implementation of the deflate compression algorithm. deflate is used by protocols such as SSH and HTTP, and file formats like PNG, GZ and ZIP.
The deflate format involves two kinds of compression:
- Lempel-Ziv encoding, which can replace repeated content with references to one copy.
- Huffman encoding, which can encode symbols using a variable number of bits. (Compare to regular 8-bit ASCII, where 'e' and 'z' always take up 8 bits each, even though the former is often far more common).
Most compression algorithms designed for speed only do Lempel-Ziv. Some examples are Snappy, LZ4 and LZJB. They tend to be byte-orientated, processing data in multiples of 8 bits.
Variable-length encodings tend to be slower. Not only does determining the right Huffman table for particular content take time, but operating on individual bits is more work. The deflate specification describes one "fixed" Huffman table which most encoders use on their "fast" setting, but the encoder still has to shunt bits around.
ajflate does one new thing - it produces compressed content using a tailored Huffman encoding which outputs only 1-byte and 3-byte data fragments. This allows the encoder to work one byte at a time, more closely resembling the aforementioned fast encoders than a typical deflate encoder. In principle, it can be very fast.
There are tradeoffs:
- The deflate / zlib format really wasn't designed for this. It's shoehorned in. The encoder will never be quite as fast as something like LZO, and will always compress a bit worse.
- Only 7-bit values (ie those values under 128) can be encoded. This is true for many English web pages and Javascript files. It's not true for PNG images, EXE downloads, non-English text or many other things. The encoder will break out of compression mode to handle this content, so it'll "work", but the encoding will be slow and the resulting "compressed" data will be large.
- lib/ contains a standalone implementation. It's not as optimized as it could be.
- example/ contains a simple demo of the library.
- tools/ contains a slower experimental implementation, and other tools to decode and explore GZ files.
- contrib/ contains utility code from elsewhere.
The contents of tools/ are separately licensed under the GNU General Public License, Version 2.
The contents of contrib/ are derived from third-party code; see the files in that directory for specific license detail.
Everything else is licensed under the below 2-clause BSD license. For the removal of doubt, everything in lib/ is BSD licensed, even if a copy also appears in the more-restrictively-licensed tools/ directory.
ajflate was written by Andrew Francis.
Copyright (C) Grumpy Panda Pty Ltd as trustee for the AJF Family Trust. All rights reserved.
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